On the left hand side we see that the bottom heater is set to follow the top heater with a difference of -40°C. So a top heater reading of 200°C would steer the bottom heater to 160°C. (more about pre-heater).
The reflow TAL time is set to 15 seconds. This means that when reaching the highest temperature point in the curve, the oven controller will keep that temperature for at least 15 seconds. When this function is set, the oven will heat until the highest temperature is reached, regardless in which time frame. It will hold the temperature for 15 seconds in this case and then jump back to that highest point in the curve and continue the rest of the curve. (see also eC-reflow-mate hold time and eC-reflow pilot hold time).
The curve itself looks unsmooth and when performing a solder test we can see that the temperature shown in the temperature scope in the bottom part of the screen, does not follow the theoretical curve at all. What is happening?
Controlling the oven is like driving a car.
The first step we step in our curve is jump to 100°C in a few seconds. That is not going to happen. It”s like pushing the gas lever of your car when the traffic light turns green. The car is not jumping from 0Km/H to 70Km/H in 0 seconds flat. It needs time to accelerate. The oven reacts in the same way. Telling the oven to heat up, will launch a current through the Infrared lamps. This current will heat up the lamp wire which then will start to heat up the oven and the board. It will take some time.
The second section of the curve reflects a slow acceleration. After you have told your car to accelerate, you release somewhat the gas lever, giving your car time to accelerate slowly to the 70Km/H desired. The same you do with the oven.
Whenever the oven reaches the highest point of the curve depends on the type of board you are soldering. Heavy copper boards will require more time. Time is exactly the only parameter to manipulate the curve: “time”. When you do not know how a board is going to react, it is best to solder it using the external sensor and thus measuring the temperature on board and using the reflow TAL function, explained above to guarantee that you reach the reflow temperature.
Cool down, open the door.
The next point is the door open point. The temperature will never drop like we define in the theoretical curve. What we set is “hit the brakes” like we do with our car when we see a red traffic light. Only in case of the oven it means that we stop heating. From that point on the oven will cool down very slowly as he is very well insulated. From the moment that the temperature reaches the temperature set in the door open point, the door will open. The door open point is temperature controlled and not time controlled as we need to be below solder temperature before we open the door thus no components will move anymore.